This invention concerns cements, and particularly calcium aluminate refractory cements.
Refractory products known as castables are made by combining refractory aggregate, for example calcined bauxite or tabular alumina, with cement (i.e., a material which will react with water to form solid hydrated phases, yielding a solid monolithic structure when the mass has set). Castables are used by mixing with water (generally in an amount from 5 to 15% by weight of the dry material in the case of a dense aggregate, and up to about 50% in the case of a porous insulating aggregate), casting (or gunning) into place in forms, and allowing the cast material to set, just as is done with ordinary concrete made with portland cement. However, since refractory castables are intended for use at relatively high temperatures, the cement used must be more refractory (i.e., have a higher melting or softening point) and move volume stable under cyclic heating than is portland cement.
Refractory cements, like portland cements, are made by admixing selected raw materials, for example limestone and alumina or bauxite, and heating them, for example in a rotary kiln, to a temperature at which they react to form phases such as 12CaO.7Al.sub.2 O.sub.3 (C.sub.12 A.sub.7), CaO.Al.sub.2 O.sub.3 (CA), and CaO.2Al.sub.2 O.sub.3 (CA.sub.2). This clinker is then ground, for example until at least 75% passes a 325 mesh screen, to form the refractory cement.
It has been found that a clinker in which CA (i.e., CaO.Al.sub.2 O.sub.3, a distinct mineral phase) constitutes a major portion of the composition forms a very satisfactory refractory cement. However, from the CaO-Al.sub.2 O.sub.3 phase diagram it can be determined that such a clinker has a melting point of about 1600.degree.C. Often, it is desired to have a refractory cement which is more refractory than one made with CA clinker.
It is known to make a clinker of higher alumina content so that it contains both the CA phase and the CA.sub.2 phase. From the CaO-Al.sub.2 O.sub.3 phase diagram, it can be seen that such a clinker is more refractory than one which is solely CA. However, the presence of CA.sub.2 in the clinker tends to reduce the strength at intermediate temperatures and may slow the hardening rate of the concrete in which it is used. Since refractory castables are often used to repair furnaces in the middle of a campaign, it is highly desirable that they harden as rapidly as possible so as to minimize the time the furnace is out of use for repair. Thus, cements in which the setting ingredient is substantially all CA are preferred for their greater strength and more rapid hardening. It is also known to increase the refractoriness of a calcium aluminate cement by adding finely divided alumina to the clinker after it is made.
Nearly all cements show a decrease in strength upon heating from room temperature to elevated temperatures. This is due to decomposition of the hydrated cementitious materials formed during room temperature hardening. At higher temperatures (e.g., at temperatures of from 1200.degree.C upwards), the strength of the castable increases again due to formation of a ceramic bond. Thus, concretes made with refractory cements exhibit relatively low strengths at intermediate temperatures (i.e., in the region of 1000.degree. to 1100.degree.C). The strength at intermediate temperatures therefore represents a critical property, since this is the weakest point in the refractory. (It will be understood that in most applications, for example in a furnace wall, there will be a temperature gradient through the thickness of a cast section, and that the weakest point will be the point which is at an intermediate temperature, for example around 1000.degree.C.)
It has been suggested, in U.S. Pat. No. 3,617,319, to overcome this problem of low intermediate strength in cements made with clinker and added alumina by including a flux such as cryolite (AlNa.sub.3 F.sub.6) which will melt at intermediate temperatures and help create a ceramic bond. However, such cements have the shortcomings that addition of the flux makes them less creep resistant, and also reactive with silica, forming silicon fluoride gas. Therefore, they cannot be used with refractory aggregates containing silica, for example the very commonly used aluminosilicate aggregates. In other words, cements containing fluoride fluxes should only be used with low silica aggregates.
The present invention provides a refractory calcium aluminate cement of good refractoriness, rapid hardening, and good strength, both at room temperature and at intermediate temperatures.